I have .20 Walther barrels for it but no .22's
The .20 is a better choice than a .22 in my opnion. My Shadow with a 15 inch .20 barrel shoots 11.2's at 790's / 800 fps, with a .22 in a Gamo you won't get much over 680's.
In fact a customer just brought over a Shadow 890 in .22 for a .20 barrel conversion and we put some 14.3's over the crony, 620's was all it would do.